1. Field of the Invention
The present invention relates to single-sealed metal vapor electric discharge lamps such as small-size metal halide lamps, and more particularly, to single-sealed metal vapor electric discharge lamps with an improved bent portion of the electrode rod.
2. Description of the Related Art
Conventionally, for outdoor lighting and plant lighting, high-intensity discharge lamps (HID), that is, high-pressure metal-vapor electric discharge lamps have been used. Recently, high-pressure metal-vapor electric discharge lamps have been gaining popularity the use of indoor lighting of low shop ceilings.
The popular use of high-pressure metal-vapor electric discharge lamps is attributed to the downsizing of the light emission tube of the discharge lamp, the external lamp tube material quartz, as opposed to hard glass, has higher heat resistance, and the reduced overall lamp size. In addition to this, because the high-pressure metal-vapor discharge lamps can utilize conventional properties of high efficiency, high color rendering, high output, and long life, the use of the high-pressure metal-vapor discharge lamps in place of incandescent lamps and halogen lamps can reduce electric consumption.
In particular, the metal halide lamp provides superiority or high efficiency and high color rendering compared to other discharge lamps. These attributes are especially suitable for lighting of displayed products, and thus their popularity has been rapidly increasing.
However, employing the conventional double-sealed envelope construction for downsizing the light emission tube not only requires time and labor in forming but also increases the sealed portion size, thus increasing the overall size. Moreover, it has a drawback that heat loss from the light emission tube increases through these sealed portions.
For this reason, with this kind of small-size lamps, the compression-sealed portion is formed in the shape of the light emission tube on one side of the envelope only, to which a pair of electrodes are sealed; that is, single-sealed construction is employed.
The single sealed configuration achieves smaller heat loss a compared to the double-sealed form envelope, thereby permitting improvement of light-emission efficiency. In addition, no extra time and labor is required for forming, and the sealed portion that tends to increase the size relatively as compared to the electric discharge space is reduced to only one; reducing the whole lamps size.
The single-sealed lamp of this kind has a pair of electrodes guided to the electric discharge space from one sealed portion. Consequently, a pair of electrode rods tends to be arranged in parallel to each other, increasing the possibility of electrical discharge between electrode rods. That is, electric discharge in the discharge space tends to occur between a pair of electrodes where the distance between electrodes is the shortest and also at the place susceptible to the condition of easy electrical discharge. For this reason, in the single-sealed lamps, electric discharge sometimes occurs at the electrode rods since the difference in electrode-to-electrode distance and electrode coils which are formed at the tip ends of these electrode rod is small.
Such electric discharge at the electrode rods not only accelerates blackening due to scattering of electrode rod material over the arc tube but also breaks the electrode rods early.
To avoid this phenomenon, the electrode rod tip ends are bent closer to each other and the tip ends of these bent portions have electrode coils. This makes the distance between electrode coils shorter than that between electrode rods, allowing the discharge to occur surely between electrode coils and preventing generation of discharge between rods.
However, when the electrode rod tip ends are bent, an excessively small or large bend angle reduces difference between the clearance at the bent portions and the distance between base ends of electrode rods and it becomes difficult to make clear difference between distance between electrode coils and that between electrode rods, cancelling the effect of prevention of discharge between rods.
Too small of a curvature radius of the bent portion causes damage to the bent portion during bending resulting in breakage and lower yields. Furthermore, there is a problem that cracks generated during bending grow in service and cause breakage in the bent portion, eventually dropping electrodes.